Theresa May has sought to reassure EU leaders that Britain remains "unconditionally" committed to the collective defence of Europe, despite leaving the bloc.

The Prime Minister, in Estonia for an EU digital summit, said the UK would continue to provide assistance to states which were victims of armed aggression, terrorism or natural disasters.

Her comments echo her Florence speech last week, when she held out the prospect of a comprehensive security agreement with the EU in her effort to unlock the stalled Brexit negotiations.

However British hopes that her initiative would enable the talks to move swiftly on to the second phase - including a post-Brexit trade deal - received a fresh setback after European Commission president Jean-Claude Juncker said it would require "miracles" if they were ready to start before the end of October.

"By the end of October we will not have sufficient progress. At the end of this week I am saying that that there will be no sufficient progress from now until October unless miracles would happen," he told reporters at the summit in Tallinn.

Mr Juncker's comments come after the EU's chief negotiator Michel Barnier said on Thursday it could be "months" before the issues of Britain's "divorce bill", future citizens' rights and the Irish border were settled and the negotiations could move forward.

Mr Barnier is expected to meet Brexit Secretary David Davis in Brussels for the start of a fifth round of negotiations on October 9, but Mr Juncker's comments suggest a breakthrough in those discussions is unlikely.

Arriving in Tallinn, Mrs May insisted she was "pleased" with the progress the negotiations had been making towards a "deep and special partnership" with the EU once the UK has left.

Earlier, addressing British troops serving with the Nato battlegroup in Estonia, the Prime Minister underlined her commitment to the future security of Europe.

While she carefully avoided the apparent threat to withdraw co-operation in her letter triggering the start of the Article 50 talks on Britain's withdrawal, she nevertheless underlined the UK's contribution as one of the most capable military powers in the EU.

"While we are leaving the European Union, as I have said many times, we are not leaving Europe so the United Kingdom is unconditionally committed to maintaining Europe's security," she said.

"Our resolve to draw on the full weight of our military, intelligence, diplomatic and development resources to lead international action with our partners on the issues that affect the security and prosperity of our peoples is unchanged.

"And our determination to defend the stability, security and prosperity of our European neighbours and friends remains steadfast."

Britain currently has 800 troops stationed in Estonia and leads the battlegroup which forms part of Nato's "enhanced forward presence" in eastern Europe designed to deter Russian aggression.

Addressing the troops at the Tapa military base, the Prime Minister said they stood ready to respond to any threat from Moscow.

"Russia's continued aggression represents a growing danger to our friends here in Estonia as well as Latvia, Lithuania and Poland, and our response must be clear and unequivocal," she said.

"When a nation like Russia violates the rules-based international order that we have worked so hard to create, we must come together with our allies to defend that international system and the liberal values, human rights and the rule of law by which we stand."